Actin is a highly conserved protein present in virtually all eukaryotes. Actin has been implicated as a major contractile protein that, along with myosin, mediates many intracellular movement processes. It contains one residue of the rare amino acid, 3-methylhistidine. This methylation has been conserved throughout evolution, and thus has been presumed to be important. However, its biological significance is still not known. The research described in this application will address the biological role of actin methylation. This will entail characterization of the localization of the 3-methyhistidine residue in the Dictyostelium actin primary sequence. Also to be assessed is whether the actin methyl moiety is a static or dynamic feature of the actin molecule; that is, whether it turns over. If it does turn over, it may serve a regulatory function, and studies will be conducted to determine if the extent of methylation varies with the state of cellular activity. Furthermore, the actin methyltransferase will be purified and characterized. Screening for mutants in actin methylation will be undertaken in order to confirm the physiological consequences of this protein modification. The effect of methylation on the properties of purified actin will be assessed in terms of polymerization, on the properties of purified actin will be assessed in terms of polymerization, supramolecular assemblies, and the interaction of actin with myosin and other proteins.